Xine
Xine is a multimedia playback engine for UNIX released under the GNU General Public License. It can play CDs, DVDs, and Video CDs, as well as common computer multimedia formats like AVI, WMV, MOV, and MP3.Xine consists of a shared library, several plugins, and a front-end player application. Many other programs use the Xine library for multimedia playback.
MPlayer is its free software colleague.